On Friday morning, a coder turned up a surprising feature in OS Mavericks, a chunk of code that seemed to reset idle time whenever the computer sensed movement. Immediately, the concern spread to Twitter. Did that mean the iSight camera was always on, opening up the same privacy concerns that have dogged the next-generation Kinect? Tests seemed to confirm the suspicion, when moving around in front of the computer was shown to stave off sleep mode. As long as the computer sensed movement in front of the screen, it wouldn’t go idle. The explanation turns out to have less to do with privacy than with clever use of sensors. Further tests from curious Twitter followers revealed the trick wasn’t working through the iSight camera but through the light sensor placed just to the left.